Conventionally, as a brake mechanism for a vehicle, for example, servo units using a negative pressure booster or a hydraulic booster are known. As this kind of a servo unit, in recent years, electric servo units using an electric motor as boosting sources have been disclosed (for example, see Patent Document 1).
The electric servo unit disclosed in this Patent Document 1 is configured as an integrated unit including a main piston that is advanced and retreated by operation of a brake pedal, a cylindrical booster piston that is fitted outside the main piston relatively displaceably, and an electric motor for moving forward and backward the booster piston.
In this case, the main piston and the booster piston are arranged as the pistons for a master cylinder, and the respective front ends are located in the pressure chamber of the master cylinder. Therein, an input thrust force applied to the main piston from a brake pedal and a booster thrust force applied from an electric motor to the booster piston generate a brake hydraulic pressure in the master cylinder.